Method of lubricating a gas turbine engine



3,000,827 METHOD OF LUBRICATING A GAS TURBINE ENGINE Edwin S. Moler,Jr., Wilmington, Del., assig'nor to Her-v cules Powder Company,Wilmington, Del.,' a corporation of Delaware I No Drawing. Filed Nov.12, 1958 Ser. No. 773,171 3 Claims. (Cl. 252--56) This invention relatesto a method of lubricating gas turbine engines. More particularly, itrelates to a method of lubricating a gas turbine engine which comprisesmaintaining at the points of wear thereof a film of an ester ofdipentaerythritol and an alkanoic acid selected from the groupconsisting of alkanoic acids of 5 to carbon atoms and mixtures ofalkanoic acids having from2 to 12 carbon atoms in proportions to providean average chain length of from 5 to 10 carbon atoms.

With the development of gas turbine aircraft engines which must operateat peak efliciency at extremely high nite ,tates Patent 26 parts ofactivated carbon,- and 75 parts of toluene. The mixture was agitatedvigorously and heated to 147 C. where it was held for 4 hours at whichpoint 67 parts of propionic acid was addedto compensate for that whichwas being lost. The temperature was then increased to 185 C. over aperiod of 3.5 hours. It was held there for one hour and then increasedslowly to 200C. over a periodof 3 hours and 15 minutes. The refluxcondenser was removed and the temperature was slowly increased to 237 C.over a period of 2.5 hours and washeld: at this point for 45 minutes.The reaction mixture was then cooled. Excess acids were next strippedofiusing a nitrogen sparge at a minimum pressure of 2.3 mm.,- a maximumstripping temperature of 215 C. was reached in 2'hours and the mixturewas held there for 30 minutes.

altitudes there has developed needs for lubricants which i areeificacious for long periods of time at the high temperatures at whichsuch engines operate. Up to the present time the gas turbines haveoperated at maximum bulk oil temperatures of about 300 F. and certainsynthetic lubricants, particularly dibasic acid esters such as dioctylsebacate, have been used successfully for several years. However, thedibasic acid esters which lubricate satisfactorily at a bulk oiltemperature of 300 F. have not been found satisfactory under the higheroperating temperatures of the newer gas turbine engines.

As a result, many other materials of synthetic origin have been examinedas possible lubricants in the new gas turbine engines. Although thereare many materials which have looked promising in laboratory evaluationof physical characteristics such as pour point, flash point, etc.,actual performance of such materials in engine testing has beendisappointing because of excessive sludge and coke deposits. Forinstance, esters of pentaerythritol and alkanoic acids have manyproperties that recommend them as lubricants for gas turbines. However,when this class of esters is employed in a gas turbine operating at anoil-in temperature materially above 300 F., there is a substantialbuild-up of coke, sludge and varnish on the bearings and in the bearingcompartments of the engine.

The present invention is based upon the discovery that certain esters ofdipentaerythritol, when used as lubricants for gas turbine engines, areparticularly outstanding with respect to their ability to maintainengine cleanliness even at bulk oil temperatures in excess of 300 F.

In accordance with this discovery the invention relates to a method oflubricating a gas turbine engine which comprises maintaining on thebearings and other points of wear of the engine a film of adipentaerythritol ester of an alknnoic acid selected from the groupconsisting of alkanoic acids having from 5 to 10 carbon atoms andmixtures of alkanoic acids having from 2 to 12 carbon atoms inproportions to provide an average chain length of from 5 to 10 carbonatoms.

The preparation of a dipentaerythritol ester of the type hereinabovedefined is illustrated by the following procedure in which parts are byweight:

Into a reaction vessel equipped with a thermometer, stirrer, refluxcondenser and water trap, there was placed 254 parts ofdipentaerythritol, 1050 parts of a mixture of acids having the followingcomposition:

Other properties of the aboveester were "as followsf Specific gravity,25/25 C 1.0137 Refractive index, 20 C 1.4567 Flash point, F 572 Flamepoint, "F 617 Pour point, F -50 Evaporation loss, 400 F 6%. hrs.,percent 1.6

: Viscosity, cs.: I At 400 F 1.5

' At 210 F 3,8.9

At 100 F 62.9

At 0 F 5000 From the various properties listed above, it is apparentthat the ester has satisfactory flash, flame and pour points for use asa lubricant at high temperatures and that its evaporation loss at 400 F.is relatively small. However, when the ester was evaluated as alubricant in a gas turbine operating at a bulk oil temperature of 350 F.it was found surprisingly that the cleanliness of the engine wasmaintained to a remarkable degree over a relatively long period of time.

As a demonstration of the invention, the above ester (admixed with 0.5%phenothiazine by weight of ester) was employed as the lubricant for a 65hour run in a Pratt and Whitney J57P29 gas turbine following theprocedure of US. Air Force Specification MIL-L-9236. Briefly, in thistest the tank of the test engine is filled with the test lubricant andthe engine is run through a series of 5-hour cycles at 350 F. oil-intemperature. Each five hours the oil consumption is measured and oiladded as necessary. The filter is changed each 25 hours, with the filterweight pick up being measured. Upon completion of the run the engine isdisassembled, inspected and photographed. It is then compared to areference engine to determine the relative deposit rating of the testlubricant. The reference engine is one which has been run by theprocedure of MIL-L-7808 using an approved lubricant at an oil-intemperature of 300 F. (See Wright Air Development Center Tech Note 572.12.) In the following table of ratings of the various areas of theengine the symbol A denotes that the test engine was considerablycleaner than the reference engine, the symbol B denotes that the testengine was slightly cleaner than the reference engine and the symbol Cdenotes the same degree of cleanliness as the reference engine.

Patented Sept. 19., 1961 A. Main bearings:

Bearing Compartments:

#1 #2 #3 #4 #5 #6-sump C. Accessory compartments:

"N (10 speed) N (hi speed) D. Breathers' As seen from the precedingdata, the test engine was quite clean. There was an almost complete lackof sludge and varnish, the ratings less than A being the result ofslight despositions of coke. Additionally, there was no visiblecorrosion.

The dipentaerythritol esters employed in the invention can be preparedby the complete esterification of dipentaerythritolwith an alkanoic acidor mixture of alkanoic acids as hereinbefore described. Specific estersuseful in the invention are, for example, dipentaerythritolhexaheptanoate, dipentaerythritol hexavalerate and dipentaerytbritolhexacaproate. Mixed esters useful in the invention can be prepared frommixtures of all (anoic acids such as acetic, propionic, butyric,caproic, caprylic, heptanoic and pelargonic, provided the mixture ofacids has an average chain length as prescribed. W The esters,"oftsetse, may be blended with one or more additives "to further enhancetheir inherent characteristics.

Such additives include, for example, antioxidants, pour pointdepressants, detergents, viscosity index improvers and th'likeT w i WhatI claim and desire to protect by Letters Patent is:

1. A method of lubricating a gas turbine engine which comprisesmaintaining on the bearings and other points of wear of the engine a ofa dipentaerythritol ester of an alkanoic acid selected from the groupconsisting of alkanoic acids having from 5 to 10 carbon atoms andmixtures of alkanoic acids'having from 2 to 12 carbon atoms inproportions to provide an average chain length of from 5 to 10 carbonatoms.

2. A method of lubricating a gas turbine engine which comprisesmaintaining on the bearings and other points of ear 'ofi'tlie e ngifie afilm of a'dipentaerythritol ester of mixed 'alkanoic' acids having from2 to 12 carbon atoms in proportions to provide an average chain lengthOfflOm 5 to '1'0' carbonatoms'in the mixture.

3. A method of lubricating a gas turbine engine whichcomprisesni'ainfainingbn the bearings and other points of ,vvear of theengine a'film of a dipentaerythritol ester of mixed alkanoic acidsselected" from the group consisting of propionic acid, caprylic'acid andcaproic acid in proportions to provide an average chain length of about6 carbon atoms the mixture."

M rqu e Cite? n e fi 9 thislpatent UNITED STATES PATENTS Q ER R F EN EJLubrication Engineering, August 1957, pages 454-

1. A METHOD OF LUBRICATING A GAS TURBINE ENGINE WHICH COMPRISESMAINTAINING ON THE BEARINGS AND OTHER POINTS OF WEAR OF THE ENGINE AFILM OF A DIPENTAERYTHRITOL ESTER OF AN ALKANOIC ACID SELECTED FROM THEGROUP CONSISTING OF ALKANOIC ACIDS HAVING FROM 5 TO 10 CARBON ATOMS ANDMIXTURES OF ALKANOIC ACIDS HAVING FROM 2 TO 12 CARBON ATOMS INPROPORTIONS TO PROVIDE AN AVERAGE CHAIN LENGTH OF FROM 5 TO 10 CARBONATOMS.